The invention relates to a coordinate-measuring instrument having a vertically guided displaceable measurement slide, a positioning drive for the measurement slide, and a device to compensate for the weight of the measurement slide.
In prior practice, counterweights have been employed to provide weight compensation for the so-called Z-axis of coordinate-measuring instruments, i.e., for the vertically guided measurement slide. Coordinate-measuring instruments with this type of weight compensation are illustratively described in West German Patents Nos. 2,613,451 and 2,943,431. However, counterweights have the disadvantage of doubling the mass of the vertically displaceable part. The machine must therefore be built more rigid in order to reduce to a minimum any deflection to which horizontal guides of the instrument might otherwise be subjected. Furthermore, larger forces are required for accelerating the measurement slide. In addition, a counterweight system dictates a vertical installation, as the only possible construction.
It has also been proposed to provide a pneumatic cylinder for weight compensation, instead of a counterweight. Such coordinate-measuring instruments are described in U.S. Pat. No. 4,207,680, in published West German Unexamined Application No. OS 3,119,228 and in West German Gebrauchsmuster No. 8,609,423. While this solution has the advantage of a smaller mass and permits any desired installed location, it nevertheless requires high construction expense since a pressure-fluid unit, possibly an accumulator for a relatively large amount of air and a pneumatic control circuit, are required in order to maintain constant pressure in the weight-compensation cylinder.
It is furthermore known from U.S. Pat. No. 3,818,596 to effect weight compensation by a coupling which is adjustable with respect to transmitted force and which is arranged between a continuously rotating drive motor and the vertically guided part. In such solutions, the drive motor itself completely assumes the weight of the guided part or even overcompensates for it, and disturbing source of heat is necessarily involved.
It is furthermore known to use a so-called spring motor to compensate for the weight of a vertically displaceable machine part. Such a spring motor uses a spiral spring, wound in the manner of a watch movement. To compensate for the relatively large variation of spring force within the working range of the spring motor, the guided part is, as a rule, coupled to the spring motor by a so-called Archimedes' spiral. This solution is also relatively expensive. Furthermore, such spring motors are not very quiet in operation since non-linear variations in spring force occur within the working range of the spring motor.